The present invention relates to acousto-optical (A-O) modulators and, more particularly, to a piezo-optic (P-O) modulator in which light is totally internally reflected from a shallow phase grating created at a modulating surface.
The increasing use of lasers in a wide variety of high speed application (printers, communication devices, etc.) has resulted in a need for modulating the amplitude, phase, frequency and/or direction of the laser beam at megahertz to gigahertz frequencies. This need has been met, to some extent, by developing high quality optical materials whose properties can be altered by applying an electrical or magnetic field so as to produce an interaction with an optical wave projected through the material. Initial efforts were directed to electro-optic (E-O) light modulators which ultilized the principle that an electric field applied to a certain group of crystals would alter the refractive index of the crystal. This approach, while feasible, has two main drawbacks: the scope of suitable crystals materials is very limited and the crystals are prone to optical damage.
Acousto-optical devices, on the other hand, utilize the principle that the refractive index of a relatively broad range of materials can be modulated by generating an acoustic strain field within the material. This is generally accomplished by affixing a transducer to a surface of the modulator. The transducer then converts electrical signals into propagating acoustic waves which interact with optical waves at a volume of intersection within the material.
In contrast to E-O modulators, A-O devices can thus be any one of a broad range of materials chosen for their optical properties. They also would not generally be prone to the optical damage inherent in using the E-O crystals.
Prior art A-O modulators, typified by the embodiments shown in U.S. Pat. Nos. 3,731,231; 3,800,303; 3,617,931 and 3,938,881, are not suitable for amplitude modulation of light at rates of 100 MHZ and above. The restrictions on the modulation rates achievable in these prior art devices derive from the physical nature of the propagating strain fields created within the interaction medium. These fields are propagated sequentially into the medium so as to establish an acoustic diffraction grating at a desired location. To maximize modulation rates, the acoustic wave and the optical wave must both be precisely focused so that a small interaction length is obtained. The rise time of the diffracted light is equal to the time required to establish the grating across the width of the light beam. Since rise time is a function of the transducer bandwidth and the width of the interaction cross-section of the light with the acoustic wave, this finite time interval imposes a modulation limit on these devices.
Since prior art A-O modulators utilize focused acoustic waves, additional disadvantages exist. The focusing elements are difficult to manufacture and to position; high acoustic power densities in the interaction region are also required.
Various other approaches have been examined to achieve higher A-O modulation rates. William Chang in U.S. Pat. No. 3,655,261, discloses a method of confining the light/sound interaction length within guided waves in thin film structures. Manhar L. Shah in an article published in the Applied Physics Letters Vol. 23, No. 10, November 1973, describes a fast acoustic diffraction type thin film optical waveguide modulator. This guided wave technology, while improving diffraction efficiency somewhat, is still subject to several problems. The optical wave must be coupled in and out of the guide, and the confinement of light to the thin film generally requires careful mode control of the incident light and creates high power density in the film.
Another approach has been to explore the effects of using the so-called total-internal-reflective (TIR) principle. Briefly stated, a light wave traveling in a bulk optical material is totally reflected from a modulating surface at a high angle of incidence. At the modulating surface, a shallow phase grating has previously been formed either electro or acousto-optically. E-O TIR modulators have been developed by Scibor-Ryliski and disclosed in Electronic Letters, Vol. 9, pp. 309-310 (1973) and Vol. 10, pp. 4-6 (1974). See also U.S. Pat. No. 4,066,338 by Hattori et al. In these modulators, light propagates in an electro-optic crystal and the phase grating is formed by attaching inter-digital electrodes at the modulating surface. While this approach is relatively inexpensive and is capable of very fast rise times, it suffers from two problems: crystal materials appropriate to this device are prone to optical damage; and light reflected from the modulating surface contains undesirable wavefront phase irregularities because of the presence of the inter-digital electrodes.
An A-O TIR structure has been developed by Kramer, Araghi and Das and described in a paper read before the 1976 IEEE/OSA CLEOS Conference. In this device, a Rayleigh acoustic wave is propagated sequentially along the surface of a bulk material forming a shallow phase grating at this modulating surface. The modulator is positioned so that the focused beam is totally internally reflected from the acoustic propagation surface. While this device provides greater efficiency than the A-O thin film devices previously described, optimum efficiency is limited by the finite Rayleigh wave propagation time. Hence, this device also has a relatively long rise time, hence, modulation limitations.
The present invention is distinguished over the prior art A-O devices in that the alternating regions of the strain field comprising the shallow strain diffraction grating propagate in parallel (rather than sequentially) across a light beam and are produced by an external stress (P-O effect) instead of an internal stress (A-O effect) associated with a propagating elastic wave. The light beam is directed so that it is totally internally reflected at, and interacts with, the strain diffraction grating. This configuration permits much higher modulation rates than previously thought possible. A further distinction over the prior art is the optical isolation of the transducer elements to prevent possible differential phase shifts from occurring if the light beam were reflected from those elements. The invention is, of course, distinguishable over the prior art E-O modulators in that the interaction medium is not of the limited class of crystals required for the E-O type modulator.